Cillizza's point was underscored by two Democratic senators — Mark Warner (Va.) and Mark Begich (Alaska) — declaring their support for gay marriage late on Monday, joining a rapidly growing number of Democratic politicians and Sen. Rob Portman (R-Ohio). The main evidence for the seemingly inevitable defeat of the anti-gay-marriage argument, though, can be found in the opinion polls.

Cillizza, making his case (this time) in three charts, notes that it's not just that gay marriage now has at least plurality support in almost all recent surveys — and 58 percent backing in a stunning Washington Post/ABC News poll — but that younger voters are overwhelmingly supportive and "every generation is growing more accepting of the idea as they age." The Week's Taegan Goddard warns supporters of gay marriage against counting their chickens before they hatch, but he also quotes a new Pew Research survey:

The rise in support for same-sex marriage over the past decade is among the largest changes in opinion on any policy issue over this time period. [Pew]

Proponents of codifying "traditional" marriage (one man, one woman) are struggling to explain why the polls have turned against them, after a long string of high-profile victories at the ballot box, says Will Saletan at Slate. Here are six possible explanations for why Americans seem to be rapidly abandoning their opposition to same-sex marriage:

1. Most Americans now have a gay friend or relativePart of the switch is driven by demographics, but in the new Pew survey, 28 percent of gay-marriage supporters (representing 14 percent of the public) say they've had a change of heart on the issue. They cite a range of reasons — they've "grown more open" with age, or think the government should stay out of the marriage business, or they just think gay marriage is inevitable — but the biggest reason, cited by 32 percent of converts, is that "they know someone — a friend, family member, or other acquaintance — who is homosexual."

A CNN/ORC International poll released Monday backs that up. In the survey, 57 percent of respondents say they have a family member or close friend who is gay or lesbian, a 12-point uptick since 2007. That includes Chief Justice John Roberts. "The number of Americans who support same-sex marriage has risen by almost the same amount in that time — from 40 percent in 2007 to 53 percent today — strongly suggesting that the rise in support for gay marriage is due in part to the rising number of Americans who have become aware that someone close to them is gay," says CNN polling director Keating Holland. "Some people have recently taken to calling it the 'Rob Portman effect.'"

2. The anti–gay-marriage arguments aren't aging wellThe arguments for confining legally recognized marriage to the union between one man and one woman tend to come down to upholding tradition, protecting the institution of marriage, and encouraging procreation in stable families. Similar arguments were made against allowing women to own property and mixed-race marriage, says Kurt Eichenwald at Vanity Fair, and the secular arguments against gay marriage are similarly "ridiculous hooey."

The argument by the anti-gay-marriage crowd is so absurd, so internally contradictory, and so awash in unproven assertions that it is difficult to take it as anything more than a construct cobbled together by people who just don't like those people.... Bottom line here is that the "marriage debate" is a debate about nonsense. No one — not a conservative, or liberal, or whatever — can stand back and "define" what marriage means. Other people's marriages have nothing to do with mine; whether my neighbors are divorced, or gay, or widowed will not lead me to change anything about how my wife and I deal with each other or how we raise our children. [Vanity Fair]

Same-sex marriage opponents, of course, find this characterization of their views unfair. "Those who believe what every human society since the beginning of the human race has believed about marriage, and is clearly the case from nature itself, will be regarded, and treated, as the next class of bigots," says Archbishop Salvatore Cordileone of San Francisco. "That's untrue, and it's not kind, and it doesn't seem to lead to a 'live and let live' pluralism."

3. Popular cultureIf you want to know why gay-marriage opponents are losing, "here's the answer: Television," Democratic media consultant Doc Sweitzer tells The Washington Post. And it's not just shows like Will and Grace, featuring gay main characters and their normal-life travails.

[TV is] the greatest socializing tool of all time. Archie Bunker changed attitudes about race and the generational divide. Gays are portrayed in all kinds of shows in a positive light, from Law and Order to CSI. They are shown as people who just want to live their lives. [Washington Post]

This is one area where opponents and proponents of gay marriage actually agree. "A lot of people are changing their minds because there's been a full-court blitz by the popular culture, by elites... to intimidate and to cower people into no longer defending marriage," Gary Bauer, the president of American Values, told Fox News on Sunday. Younger voters, especially, "have been subjected to a drumbeat of support for it from the news media, entertainment media, and higher education for literally as long as they can remember," says the Family Research Council's Peter Sprigg.

4. The military led the way (again)"It's an odd fact for Democrats to have to face," says Chris Weigant at The Huffington Post, "but the military is leading on the issue" of gay marriage "in a similar way as they were forced to lead on desegregation." Since the repeal of the "don't ask, don't tell" policy, same-sex spouses and partners "are gaining a more-equal status with military wives and husbands as time goes on." And this is having "ripple effects" on broader society.

5. Corporate America is on board, tooThe political and legal worlds are just coming around to accepting gay marriage, in fits and starts, "but major American corporations have already heard the arguments — and in their own way, decided it's okay," says John Harwood at CNBC. And it's not just Starbucks. You can still legally fire or refuse to hire someone for being gay in 29 states, but in its most recent survey of 688 major employers, the gay-rights group Human Rights Campaign found that "99 percent prohibit discrimination based on sexual orientation and 89 percent offer health benefits to domestic partners."

6. The polls are wrongFaced with surveys showing broad and growing support for gay marriage, some social conservatives are simply arguing that the polls are wrong — or "skewed," as Gary Bauer said on Sunday — and "oversell the actual amount of support for gay marriage, say Aaron Blake and Scott Clement at The Washington Post. And they have a point. A 2010 study by NYU political scientist Patrick Egan found that gay marriage performs about 7 percentage points worse at the ballot box than in the preceding polls, perhaps because "people don't want to come off as intolerant of gay people."

National Organization for Marriage political director Frank Schubert points to an Election Day 2012 poll for his organization by GOP pollster Kellyanne Conway, who found that "60 percent of American voters who actually cast ballots in the last presidential election believe that marriage is the union of one man and one woman," including 86 percent of Republicans and even 40 percent of Democrats. "I am still waiting for the newspaper headline that says, 'Large Percentage of Democrats Oppose Gay Marriage,'" Schubert says at National Review. "Somehow, I suspect it will never appear."

Gay-marriage foes also point out that their record at the ballot box is 30-4 and that they barely lost in those four defeats, and predict that young voters will change their mind as they age, says Slate's Saletan. But much of the opponents' data is outdated, and that matters in a case like this, where "a social issue has done such a dramatic about-face in so short a period of time," says Rick Ungar at Forbes. That, more than anything, "reveals how they — and those who share their prejudice — have already lost the war."